In gas turbines, thermally induced stresses have always led to cracking in turbine nozzles. Due to the harsh environment, previous field history has shown cracking along the engine axial (chordwise) direction of nozzle airfoils. Should a crack propagate through the entire length of an airfoil, such that the airfoil fails catastrophically, large pieces of the nozzle might dislodge and move downstream into a turbine's rotating hardware. The subsequent damage to the turbine's hardware (both rotating and static) would be both extreme and costly.
In doublet or triplet nozzle designs (2 or 3 airfoils per nozzle segment, respectively), the increased number of airfoils provides a certain amount of insurance against catastrophic failure through the redundancy of multiple load paths. However, with a singlet (single vane segment) nozzle (1 airfoil per segment), if not retained at both platforms, a large section of nozzle, airfoil and/or platform, could be lost into the flowpath, if the airfoil were to crack completely in two.
A typical practice includes stage 1 nozzles positively attached at the outer retaining ring only, which provides axial, radial, and circumferential restraints. At the inner rail of the nozzle, only axial restraint is provided through contact at the nozzle chordal land seal. This chordal land seal concept allows the large transient radial growth differentials, while allowing the nozzle to rotate about the outer retaining ring hook due to axial growth differentials between the inner and outer turbine cases.
A review of aircraft turbine engine designs show positively attached, yet mechanically/structurally compliant designs usually consisting of multiple plates of thin metal attached directly to either end of the nozzle segment in question. A similar attachment scheme would not be feasible for a land-based turbine of this size due to the large differences in mission transient growth between the inner and outer cases (axial and radial). Also, deflection compliant designs (flight weight) do not lend themselves to being robust, given the combination of harsh environment and the number of operational hours required for a land-based turbine when compared to those seen in commercial aircraft engines.